LONDON UNDERGROUND was this week considering abandoning the Camden Town crossover which caused a train to derail last year.
Engineers believe that the crossover can only be reinstated if tunnels are enlarged.
Larger tunnels would allow the cant on curved sections of track to be increased, reducing the risk of another derailment.
The crossover, south of Camden Town station, allows trains from the Bank and Charing Cross branches of the northern line to switch between its Edgware and High Barnet branches.
Northern line trains have not used the crossover points at Camden Town since a train derailed there in October last year (NCE 23 October 2003).
Instead, Edgware branch services have been restricted to the Bank branch, while High Barnet trains have been confined to the Charing Cross branch.
The derailment caused the two rear carriages to crash into a tunnel wall, injuring seven people.
Trains have always had to slow down to 20mph to tackle the tight bend at these points because the track is level.
Lowering one side of the track would force the train to tilt as it went round the bend, reducing pressure on the outer rail.
This in turn would require tunnel geometry to be remodelled.
'The geometry of the tunnels is the main safety issue - to reinstate the crossover service you would have to invest several million pounds to widen a 50m section, ' said a rail expert with a close understanding of the Underground.
He added that it would be easier and more reliable to keep the lines separate across the points.
An intermediate report published in November last year centred the problem on a newly installed London Underground standard switchblade. The report says that its shape helped the train wheel to climb the outer rail and come off.
The final report on the investigation into the derailment was due to published on Wednesday, after NCE went to press.